Mystery of mirages

Earlier this month on a Sunday afternoon, a giant “ancient city” appeared on the surface of Hongze Lake in Jiangsu province. But after about half an hour, the “city” was gone.

You might think that some unknown forces were at work, but it’s believed that the “city on the lake” was actually a mirage.

A mirage is a trick that light and air play with bur vision（视觉）. Normally, light travels in a straight line, but it can bend（弯曲）and change direction depending on what it passes through. This bending is called "refraction （折射）”.

If different layers（层）of air are at different temperatures, light will bend at different angles（角度）when passing through them. This can create a false image（影像）of a nearby object.

Take the “city on the lake” as an example. The layer of air near the lake is colder than the higher layers, as the lake absorbs（吸收）most of the heat. The buildings in the mirage were actually on a nearby mountain. But their image showed up above the lake after light refracted in different layers of air.

Mirages happen not only over large bodies of water, but anywhere where the ground can absorb a lot of heat. They are often seen on hot roads and in deserts. The next time you think you see a pool of water on the road on a hot summer day, you will know it's just a mirage—an image of the sky caused by bending light.

Now, glasses have become a daily necessity for some of us. So, what do you know about them? No one knows who made the first wearable pair of glasses, but experts do know that they were made in the 13th century and showed up in Italy. Early glasses were like magnifying glasses (放大镜). And most of them weren’t even made of glass! Instead, many early eyeglasses contained quartz (石英). That’s because most glass at that time was not clear, so it wouldn’t have worked well for eyeglasses.

Eyeglasses didn’t look much like modern glasses until the 1700s. That’s when people added glasses temples (镜腿) to hold glasses on their faces. Glasses also became much more common around this time, probably because reading became more popular.

People didn’t start seeing eye doctors for glasses until the 1800s. At that time, they just tried on different pairs of glasses until they found one that made their eyesight better. Eye doctors changed that. They gave an eye test for people to tell whether or not they needed a pair of glasses. Different kinds of glasses have also appeared. Sunglasses are about as old as eyeglasses. People in China started wearing sunglasses in the 12th century. These early sunglasses were also made of quartz. Today, many people also like to wear contact lenses (隐形眼镜), which were invented by a German in 1887. The first contact lenses were very uncomfortable. Luckily, today’s lenses are softer and can be worn all day. However, you should always take your contact lenses out at night.

Some people even like to wear glasses with no lenses. Called fashion glasses, they come in many shapes and sizes to fit anyone’s personal style.

The earliest form of writing appeared about 5,500 years ago. In the past, hand writing was an important subject in school. Now, many students sometimes do homework on a computer and turn in their tasks online.

Many people think typing (打字) is quick and easy to read, while writing by hand may take longer than typing. But others say writing with a pen and paper is more valuable. Writing by hand has big advantages. Research shows that when we write down ideas with a pen or pencil, we make the brain active to help us learn, remember and think. Writing by hand lets people show their creativity. No two people have the same handwriting. Writing by hand can also help people concentrate. When using a computer, people can go online or play computer games easily.

Handwriting may be pretty. However, people can get their ideas and thoughts down more quickly when they type. Typing can also make it easier for people to change. Once something is written by hand, you can’t delete it. If you want to make a change, you have to write it all over again. What’s more, some people’s handwriting may be hard to read. Typing makes everything clear, and no one will have to worry about their words being clear enough.